Downhole tractors are often employed to drive a downhole tool through a horizontal or highly deviated well at an oilfield. In this manner, the tool may be positioned at a well location of interest in spite of the non-vertical nature of such wells. Different configurations of downhole tractors may be employed for use in such a well. For example, a reciprocating or “passive” tractor may be utilized which employs separate adjacent sondes with actuatable anchors for interchangeably engaging the well wall. That is, the sondes may be alternatingly immobilized with the anchors against a borehole casing at the well wall and advanced in an inchworm-like fashion through the well. Alternatively, an “active” or continuous movement tractor employing tractor arms with driven traction elements thereon may be employed. Such driven traction elements may include wheels, cams, pads, tracks, wheels or chains. With this type of tractor, the driven traction elements may be in continuous movement at the borehole casing interface, thus driving the tractor through the well.
Regardless of the tractor configuration chosen, the tractor, along with several thousand pounds of equipment, may be driven thousands of feet into the well for performance of an operation at a downhole well location of interest. In order to achieve this degree of tractoring, forces are imparted from the tractor toward the well wall through the noted anchors and/or traction elements. In theory, the tractor may thus avoid slippage and achieve the noted advancement through the well.
Unfortunately, advancement of the tractor through a well may face particular challenges when the well is of an open-hole variety as opposed to the above-described cased well. That is, in certain operations, the well may be uncased and defined by the exposed formation alone. In such circumstances, the well is likely to be of a variable diameter throughout. For example, it would not be uncommon to see an 8 inch well expand to over 11 inches and taper back to about 8 inches intermittently over the course of a few thousand feet. Thus, without the reliability provided by a casing of uniform diameter, the tractor is left with the proposition of radial expansion to interface a changing diameter of the open hole well wall in order to maintain tractoring.
In order to ensure that the radial expansion is sufficient to maintain tractoring in an open hole, an excess of expansion forces may be employed. So, with reference to the well above for example, the amount of force imparted on the tractoring mechanisms (e.g. anchor or bowspring arms) may be pre-set at an amount sufficient to expand and drive the tractor through an 11 inch diameter section of the well. Thus, the tractor may be expected to avoid slippage when the well diameter begins to expand from 8 inches up to 11 inches.
Unfortunately, while excess expansion force may ensure tractoring through larger diameter sections of the open hole well, this technique may also lead to damaging of the tractor. For example, a conventional tractor may be equipped with anchor arms configured to withstand maximum forces of about 5,000 lbs. However, in a circumstance where the anchor arms are pre-set to operate at about 4,500 lbs. through an 11 inch diameter open hole well, forces well in excess of 5,000 lbs. may be imparted on the arms as the tractor traverses 8 inch well sections as noted above. Mechanical failure of the tractor is thus likely to ensue as a result of over-stressed anchor arms.
Furthermore, even in circumstances where the anchor arms or other expansive mechanisms are of sufficient strength and durability to withstand excess forces as noted, the exposed formation defining the well may not be. That is, in many circumstances the application of excess force may result in damage to the exposed well wall when its compressive strength is exceeded. Thus, where the formation is comparatively soft in nature, the utilization of forces adequate to drive the tractor through an 11 inch diameter well section may damage an 8 inch diameter section. Nevertheless, the utilization of excess force is often employed to help ensure tractoring through a variable diameter open hole well is achieved. As a result, the well wall often collapses or cracks in certain locations even where the tractor is left undamaged. In fact, even though technically undamaged, the tractor may be rendered inoperable with its expansion mechanism imbedded within a collapsed section of the well. In such circumstances, not only is tractoring halted, but a follow-on high cost fishing operation may be required.